Power consumption is a primary consideration for designers and manufacturers of computer systems and computer system components. This focus extends from the lowest levels of integrated circuit device design up to the computer system power supply itself. Power consumption at the system, component, and device levels is more important now than ever before for a number of reasons. Lower power consumption means longer battery life for portable computers, and improved heat dissipation characteristics for higher density integrated circuit components and computer systems. Improved heat dissipation characteristics are especially important for the smaller form factors of modern computers which leave little room for bulky cooling devices such as high air flow fans or large heat sinks.
One of the primary ways to decrease power consumption of an integrated circuit device is to lower its operating voltage. Advances in semiconductor processing technology and integrated circuit design have helped to reduce the operating voltages of many CMOS integrated circuit devices from 5 volts to 3.3 volts. Current and future generations of processing technology will provide for components which operate at even lower voltages such as 2.9 volts, 2.5 volts and 1.8 volts.
The operating voltage of a particular component is significant because it often determines the voltage swing, or high and low levels of input and output signals, associated with that component. For example, a device with an operating voltage of 3.3 volts may receive input signals and generate output signals which vary from 0 volts to 3.3 volts in magnitude. While this is not always true, such as in the case where low voltage swing transmission logic, such as GTL, is used, many integrated circuit components receive input signals and generate output signals with voltage swings determined by their operating voltage or Vcc.
While components designed and manufactured to operate at lower voltages provide reduced power consumption, it is not feasible for all integrated circuit components to be redesigned and transferred to new lower voltage processes simultaneously. Thus, a lower voltage integrated circuit component must often be capable of interfacing or communicating with older, higher voltage components in a computer system, for example. The higher voltage signals from higher voltage integrated circuit components are alternately referred to herein as "legacy signals".
One approach to addressing this issue relies on NMOS pass gates integrated on the lower voltage chip near its inputs. The NMOS pass gates reduce the legacy signal voltage and clamp it in a safe range such that it is usable by the rest of the chip. There is an issue, however, when the lower voltage process used to manufacture the chip, cannot tolerate the high voltage of the legacy signals.
Integrated circuit components fabricated to operate at lower voltages have reduced gate oxide thicknesses as compared to their higher voltage counterparts. Thinner gate oxides decrease the ability of transistors to withstand high gate-to-source voltages without experiencing either gradual or catastrophic failure. Catastrophic failure results when a voltage applied to a transistor is higher than the transistor breakdown voltage. The thinner gate oxides of lower voltage components can reduce transistor breakdown voltages significantly. Thus, the use of integrated pass gates to reduce the voltage of legacy signals is not a viable approach where the transistor breakdown voltage of the pass gate is lower than or near the input voltage range.
Another approach for interfacing lower voltage integrated circuit components with legacy signals, uses external buffer devices to decrease the voltage of a signal to a usable level before it reaches the low voltage integrated circuit component input buffer. Each external buffer used, however, introduces a signal delay. With the increased operating frequencies of many computers, and tighter timing restrictions on clock signals, this approach is not viable in many high frequency operating environments. Further, external buffers take up additional space in the computer system reducing design flexibility and adding to the overall system cost.
Thus, it is desirable to have an apparatus and method for allowing integrated circuit components with low operating voltages to receive higher voltage signals without adding unacceptable signal delays or compromising transistor reliability.